I want to build off my Arbitrage post a little bit here by talking about supply and demand.
Everyone knows about supply and demand and anyone can extrapolate and use that knowledge on the AH to make World of Warcraft gold. But this thing that doesn't jump out at people, and that is very simple once you think about it, is the demand changes on the weekends. The demands for consumables goes up dramatically on the weekends. More people play, and they want to complete their quests or goals in the time they have.
So the simple lesson is sell only on the weekends. All the stuff you farm or gather or buy during the week, save and sell it on the weekends. You not only sell it at a higher price, but your chances of it selling increase. There are more people on, and they want that new sword NOW!
I really want to stress that point. It is not just that more people are on, thus driving up the price. It is that more people are on that cannot or do not play during the week. They want that sword/enchant/gem now, and they will over pay because they do not want to wait for the price to fall, this is their only time to play.
You can make money by buying during the week, and selling on the weekend.